1. Field of the Invention
The present invention relates to a cathode ray tube having a getter for obtaining a high vacuum.
2. Description of the Related Art
A conventional cathode ray tube is processed to further increase the level of its vacuum by first exhausting a cathode ray tube having a getter, then sealing the exhaust tube, heating the getter with a high frequency induction heater to evaporate and scatter it for thereby absorbing the residual gas in the cathode ray tube.
As examples of the cathode ray tube (CRT), there are a Braun tube or a flat display panel (FDP) to be used in a television receiver or a monitor screen of a computer, or further a traveling-wave tube (TWT) to be used in a high frequency amplifier or a high frequency oscillator.
A structure of the conventional cathode ray tube will be described with reference to FIG. 1.
As shown in FIG. 1, a CRT comprises tube body 1 which serves as a vacuum vessel, cathode 2 which serves as an electron emission source, and getter 3 for augmenting the degree of vacuum.
Further, tube body 1 comprises tip tube 4 to be used as an exhaust port. Tip tube 4 is made of glass and heated by a heater, and accordingly softened and sealed, when an exhausting operation is completed. In the neighborhood of cathode 2 in the direction of electron emission from cathode 2, electron lens 6 is provided for controlling an orbit of the electron. Hermetic pin 5 is provided for impressing voltage on electron lens 6 and cathode 2.
As an electron emission source, there are used a hot cathode for emitting electrons by heating its target made of a substance which can easily emit electrons, or a field-emission type cold cathode called as a microfield emitter. The field-emission type cold cathode is manufactured by preparing a cathode cone on a conductive substrate as a sharp electron emitter of a cone shape, providing an insulation layer on the conductive substrate in such a manner to enclose the cathode cone, and providing on the insulation layer a gate layer having a hole of a submicron level which exposes the cathode cone. By providing voltage of a positive electrode to the cathode cone for the gate layer, electrons are emitted from the tip of the cathode cone. This technique is described in, for example, Japanese laid-open patent publication No.147129/1995. A device housing a getter is described in Japanese laid-open patent publication No.124502/1996. Barium is generally used as a getter and such getter is called a barium getter. This barium getter is generally composed of barium-nickel alloy so that the alloy can be stable in the tube body which has not yet become a vacuum.
Next, a vacuum device of the above CRT is described with reference to FIG. 2.
Tip tube 4 of CRT tube body 1 is connected to exhaust manifold 7 having an O ring made of rubber. For hermetically sealing the connection part of tip tube 4 and exhaust manifold 7, exhaust manifold 7 tightly holds the outside wall of tip tube 4 through the rubber O ring. Vacuum pump 9 is connected to exhaust manifold 7 through valve 8.
Since the desirable vacuum of the cathode ray tube is in a range of 1.times.10.sup.-5 Torr to 1.times.10.sup.-9 Torr, a combination of an oil diffusion pump and an oil rotary pump or a combination of a turbo molecular pump and the oil rotary pump is used as vacuum pump 9. However, since the above vacuum is hard to achive only by exhaustion through a small tip tube, it is generally known to jointly employ an absorbing function to be provided by a getter, and hence getter 3 is provided in tube body 1 and induction heating coil 11 is provided on the outside of tube body 1 as a means for heating getter 3.
High frequency induction heating coil 11 is set up to generate energy enough for evaporating and scattering getter 3. Descriptions are made with reference to heating of the getter in Japanese laid-open patent publication No.85793/1995 or No.124502/1996. Further, in some cases, an optical sensor which is used for monitoring the heating state of the getter by color is disposed in the CRT for monitoring the temperature of the getter through a transparent portion of the CRT.
For the purpose of augmenting the degree of vacuum in the tube body and reducing the exhaust time, tube body 1 is housed in heating furnace 10. Since the softening point of glass constituting tube body 1 is about 400.degree. C., heating furnace 10 is prepared so that it heats the tube body 1 at a temperature of below 400.degree. C. When tube body 1 is heated, the temperature of tip tube 4 and exhaust manifold 7 are also raised, although exhaust manifold 7 is partially held cold. If there is an extreme temperature difference in glass tip tube 4 between the exhaust manifold 7 end and tube body 1, end a crack is produced in tip tube 4. Therefore, exhaust manifold 7 is controlled so that it may not be excessively cooled.
High frequency induction coil 13 for baking(specifically heating) an electrode of electron lens 6 and electric heater 12 for sealing tip tube 4 are provided on the outside of tube body 1.
Next, the epitome for a vacuum producing method of CRT to be implemented by using the above device is described with reference to FIG. 3.
First, tip tube 4 of tube body 1 is attached to exhaust manifold 7. Then, the exhaustion of tube body 1 is started. Thereafter, tube body 1 is heated and cooled by heating furnace 10 according to a fixed temperature profile of the first half heating and the latter half cooling. In the cooling process, tip tube 4 of tube body 1 is sealed. The exhaustion of tube body 1 is continued until tip tube 4 is sealed.
The method of heating and cooling tube body 1 by heating furnace 10 according to the fixed temperature profile is described, for example, in Japanese laid-open patent publication No.32130/1992. Further, a process called "electrode baking" is often performed in the period of the above tube heating process. The "electrode baking" is a process for applying high frequency induction heating to electron lens 6 in tube body 1 to make lens 6 emit gas from the electrode thereof. The gas emitted by the "electrode baking" is generally exhausted in the above cooling process of tube body 1.
Further in the exhausting process of tube body 1, a process for evaporating and scattering getter 3 is performed immediately before sealing tip tube 4. Or, in some case, getter 3 is evaporated and scattered after tip tube 4 is sealed. Although the getter has an absorbing function for the gas other than inert gas, no absorbing function is expected for inert gas such as argon or helium.
The method of evaporating and scattering getter 3 before sealing tip tube 4 has an advantage that a part of the inert gas emitted from getter 3 can be removed by vacuum pump 9. On the contrary, when getter 3 is evaporated and scattered after tip tube 4 is sealed, disadvantageously the emitted inert gas remains as it is. In the latter case, it is known that in particular a large quantity of argon gas remains.
In the former case, evaporated and scattered getter 3 shows, immediately after being scattered, a comparatively quick absorbing function for gas other than inert gas.
Therefore, if the exhausting operation is performed for a long time without sealing tip tube 4 after getter 3 is evaporated and scattered, getter 3 comes to attract and absorb the gas other than the inert gas in vacuum pump 9, thereby causing an inverse pressure phenomenon. If the inverse pressure phenomenon takes place, the stain such as the oil of the vacuum pump transfers into tube body 1 thereby severely degrading the vacuum in tube body 1. Therefore, the sealing operation of tip tube 4 must be performed before the inverse pressure phenomenon as above takes place.
Further, in the former case, a microfield emitter is driven after tip tube 4 is sealed to test the driving condition. Particularly, the time when cathode 2 emits electrons is the period of a high vacuum of 1.times.10.sup.-9 Torr or less (hereinafter called a "high vacuum mode") produced by the absorbing function of getter 3 after tip tube 4 is sealed. In other words, as shown in FIG. 4, the microfield emitter is not driven in the period when the vacuum level is degraded by gas emitted from getter 3 (hereinafter called a "gas emission mode") after the getter is heated, and is first driven for emitting electrons after the tube has entered the "high vacuum mode".
Here, it is to be noted that with reference to the CRT which uses a cathode called an oxide cathode, a process called cathode decomposition is generally implemented such that the cathode is electrically heated in the period of relatively high oxygen density just after commencement of the exhausting operation, but this process is intended to oxidize the cathode and is not intended to cause electron emission.
As described above, since the conventional vacuum producing method of the cathode ray tube has had a problem that the inert gas remains in tube body 1, can not be removed after tip tube 4 is sealed, sealing of tip tube 4 is performed after the getter evaporation and scattering process which emits a large quantity of inert gas. However, in this case, as previously described, tip tube 4 must be sealed before the inverse pressure phenomenon appears.
On the one hand, cathode 2 of the cathode ray tube requires the advance processing for improving the electron emission characteristic thereof. The reason is that, for example, the electron emission efficiency of the cold cathode using a microfield emitter is lowered if the tip of the emitter cone made of molybdenum metal is spoiled or oxidized. As an advance processing method, there are various methods such as a method of cleaning the cathode surface by applying a heating process to the cathode in vacuum, a method of causing the self augmentation of electron emission (this process is usually called aging) by making the cathode continuously emit electrons, or a method of removing the pollution or oxide layers on the surface of the cathode through ion sputtering. If the pollution of the cathode is heavy or the cathode surface has thick oxide, ion sputtering is the most effective advance processing technique.
Further, in order to keep the cathode surface free from the compounds after removal of the pollution, inert gas is preferably used as the sputtering gas.
However, according to the conventional technique, the advance processing of the cathode is not executed during the exhausting process of the cathode ray tube, by introducing a necessary amount of the sputtering gas into the tube.